There have been known electroluminescence displays (ELD) as a light-emitting electronic display device. Constituent elements of the ELD include an inorganic electroluminescence device and an organic electroluminescence device (hereinafter, also denoted as an organic EL device). An inorganic electroluminescence device has been employed as a planar light source but requires a high alternating-current voltage to drive the luminescence device.
On the other hand, an organic EL device is composed of a luminescence layer containing a light-emitting compound between a cathode and an anode. Electrons and holes are injected into the luminescence layer and recombined to form excitons and light-emission is performed by employing luminescence (fluorescence/phosphorescence) emitted when the excitons are deactivated, so that emission is feasible at a voltage of from some volts (V) to tens of volts. Further, such an organic EL device is a self-emitting type and superior in viewing angle, exhibits high visibility and is a thin-layered complete solid device, so that it has been noted in terms of space-saving and portability.
There has been desired development of an organic EL device for a future practical use which efficiently emits a high luminescence at relatively low electric power consumption. For example, Japanese Patent No. 3093796 discloses a technique of a stilbene derivative, a distyrylarylene derivative or a tristyrylarylene derivative, doped with a slight amount of a phosphor, thereby achieving enhanced emission luminance and longer operating life of the device; Japanese Patent Application Publication (hereinafter, also denoted as JP-A) 63-264692 discloses a device having an organic luminescence layer comprised of 8-hydroxyquinoiline aluminum complex as a host compound and doped with a slight amount of a phosphor; and JP-A 3-255190 discloses a device having an organic luminescence layer comprised of 8-hydroxyquinoline aluminum complex as a host compound and doped with a quinacrydone dye.
In the foregoing techniques, when using luminescence from an excited singlet, the formation ratio of a singlet exciton to a triplet exciton being 1:3 results in the probability of forming light-emitting excited species is 25% and a light-extraction efficiency being approximately 20% leads to a limitation of external quantum efficiency (η ext) being 5%.
However, after an organic EL device using phosphorescence emission from an excited triplet was reported by Princeton University [M. A. Baldo et al., Nature 395, 151-154 (1998)], studies of materials exhibiting phosphorescence at room temperature have become more active, as disclosed in M. A. Baldo et al., Nature 403 (17) 750-753 (1998) and U.S. Pat. No. 6,097,147.
Further, a recently found organic EL device employing phosphorescent luminescence enabled to realize approximately four times higher light-emitting efficiency than prior devices employing fluorescent luminescence, so that not only development of materials but also research and development of layer constitution and electrodes of light-emitting devices. For example, there have been studied syntheses of many compounds around heavy metal complexes such as iridium complexes, as disclosed in S. Lamansky et al., J. Am. Chem. Soc., 123, 4304 (2001).
An organic EL device is an all-solid device constituted of an organic material layer as thick as 0.1 μm or so between electrodes, and achieving light emission at a relatively low voltage so that it is expected as a technique used for a next generation planar display or illumination.
Such an organic EL device has a light-emitting mechanism based on a luminescence phenomenon employing quenching of from an excited state to the ground state of an organic material. Accordingly, to emit light in the short wavelength region of blue, bluish green and the like, the band gap needs to be broadened, requiring a higher voltage to achieve excitation through such a broadened band gap.
Further, since the excited state is located at a relatively high level, damage in returning to the ground state becomes larger and its life tends to be shortened, as compared to light emission of green or red. Specifically, phosphorescence emission employing light emission from a triplet excited state exhibits markedly such a tendency.
There are various techniques to overcome the problems described above. Examples thereof include a technique of forming a constituent layer of an organic electroluminescence device, followed by its polymerization, in which bi-functional triphenylamine derivatives having two vinyl groups in the molecule are described and such compounds are used to form a layer, followed by exposure to ultraviolet rays to form a three-dimensionally cured polymer (as disclosed in, for example, patent document 1); a technique of adding a material having at least two vinyl groups to plural layers, in which a polymerization reaction is performed by exposure to ultraviolet rays or heat at the time when forming an organic layer, prior to superposing a cathode (as disclosed in, for example, patent document 2); a production method in which a radical generating agent of AIBN (azobisisobutyronitrile) is added to a mixture of a material containing a vinyl group at the end of a phosphorescence dopant and a co-monomer containing a vinyl group and polymerization is allowed to proceed during layer formation (as disclosed in, for example, patent document 3); and a production method in which a Diels-Alder reaction is caused between two molecules within the same layer to perform curing (as disclosed in, for example, patent document 4).
Any one of the foregoing techniques is a method of completing a polymerization reaction during layer formation or immediately after forming a layer (or before attachment of a cathode) but is insufficient from a practical point of view of enhancement of durability of an organic EL device, and therefore, a technique for further enhanced durability of the device is desired.
Patent document 1: JP-A 5-27 Patent document 2: JP-A 2001-29 Patent document 3: JP-A 2003-7 Patent document 4: JP-A 2003-8